DE2511557C3 - Process for making hydrophilic electrodes - Google Patents

Description

The invention relates to a method of manufacture hydrophilic electrodes that contain electromechanically convertible, active material, according to the Preamble of claim 1.

A method according to the preamble of claim 1 is known from DE-OS 2300156. With this one known method is the proportion of the Po- *> lytetrafluoroethylene at least 2% and the polytetrafluoroethylene is in the form of an aqueous emulsion before. The electrode must therefore be made in a separate operation or by increasing the temperature when applying shear forces, e.g. B. Selection rolling or pressing, to be dried.

From DE-OS 2161373 a method is for Production of the above electrodes using 1 to 10% by weight of dry polytetrafluoroethylene powder, based on the electrochemically active mass, known. In this procedure, no lubricant is used and the resulting mixture of polytetrafluoroethylene powder and active is used Material pressed into electrodes without being sintered. Those produced by this known method The electrodes provided do have a sintered element Clear mechanical elasticity, but insufficient strength.

The object of the invention is a simple one Process for the production of hydrophilic battery electrodes of high strength.

This object is achieved according to the invention by the method specified in the characterizing part of claim 1

rens steps solved.

After emerged embodiment of the invention wÄd in the production of the homogeneous mixture additional substance used, which is a higher one electrical conductivity afc possesses the active material.

During fefci the development of electrodes for Fuel cells use polytetrafluoroethylene as a binder to hold the active material has been extensively studied is polytetrafluoroethylene At the battery electrode binders, it has hardly been studied. This is due to the different Functions of electrodes for fuel cells and those for batteries. Battery electrodes must be hydrophilic to allow easy diffusion of liquid electrolyte through the active To enable electrode material. In contrast, fuel cell electrodes are said to be hydrophobic be. This is how you make the fuel cell electrode highly permeable on one side to a contact between electrode and fuel to allow however, water undiKchlia. so that a the electrode on the other hand, contacting electrolyte did not flow through them.

It is powdered active material with dry, powdered polytetrafluoroethylene in the range of 0.1 to 3 wt _.-%> preferably about 100 to 900 wt .-% "excess lubricant mixed. Examples of the lubricant are turpentine oil substitute, Stoddard solvent, propylene glycol and petroleum solvent with a specific gravity of 0.6995, aniline point of 54 ° C and a content of 65.5% by volume of paraffins, 32.0% by volume of naphthene and 2% by volume. % of aromatics. This gives a homogeneous mixture of the lubricant, powdered active material and powdered polytetrafluoroethylene. About 50 to 90% of the total weight of the mixed solids is required for lubricant; excess lubricant is used for mixing purposes to ensure an even mixture. The slurry can then be filtered, e.g. On a Buchner funnel or centrifugal filter device to remove a predetermined amount of the lubricant processing fluid and thereby reduce the excess to the desired amount. This filtered mixture or this cake is then kneaded at temperatures between room temperature and 93 ° C in a mixer of the planetary type. The lubricant at this point forms 25 to 50% of the total mixture, a range of 35 to 40% being particularly preferred.

Du.- resulting, filtered mixture, which still contains 35 to 40% lubricant, is then ground between calender rolls, the gap width depending on the desired final electron thickness on z. B. is set about 9V ₂ mm. The expanded cake is then folded or collapsed and passed through the rollers again. This pleating and then extruding treatment is repeated until the extruded tape is able to support itself without breaking. Since the polytetrafluoroethylene is flexible in the form of the very fine fibers, the slight shear action of the rollers and the resulting pressure lead to the formation of a cohesive panel. Generally five or six passes through the calender rolls are sufficient. With gap widths of more than about 1.3 cm, more diameters are

would be necessary. After each pass is easy recognizable whether the extruded material is stiff enough, to carry the sieve without breaking it. You can simply place it vertically on a rigid surface, to see if the material remains rigid or if it starts to bulge, the edge breaks. By passing through The panel through the calender honeycomb with gradually smaller gap can then be a continuous, porous surface material of the desired Gain thickness by binding the active electrode materials with PoSytetrafhioräthylen are.

The sheet is then dried to remove any residual solvent, leaving it porous remains behind. Drying can be effected by conventional methods such as air drying will. During drying, as in all of the previous stages, the temperatures are below the Sintering temperature of the polytetrafluoroethylene held. The sheet material can be pressed onto a foil or mesh pantograph during use.

While the particles of polytetrafluoroethylene touch each other during processing, shear forces form a bond between neighboring polytetrafluoroethylene teflchen the end. wherein the shear forces further tend to the particles to a very fine fibers zn fibrillate, the lubricant this Activity supported. The flat material finally formed consists of a network or latticework very fine, interconnected porytetrafluoroethyte fibers., which carries or holds the active material.

Example I.

On an Aring-F tfuart mixer, 95.5 g of zinc oxide, 2 g of mercury (H) oxide and 2.5 g of polytetrafluoroethylene powder were mixed with 200 cm of turpentine substitute for about 3 minutes. The mixture was then filtered on a centrifugal filter, with 38% by weight of turpentine oil substitute still present after the filtration. The material was then worked between two rollers for about 5 minutes at about 38 ° C, wherein it was ground by being driven between the rollers, the ground material was folded and reworked between the rollers, which was continued for five passes until the material solidified began to show. Further processing of the material was completed at this point by rolling to the desired thickness before pressing. The material, rolled to 0.66 mm thick, was then cut into 6.03 X 5.08 cm pieces; between two such pieces was sandwiched silver-plated copper mesh approximately 0.05 mm thick. This composite structure was then pressed in a pressing tool at 138 bar to form a battery plate which had a thickness of 0.66 mm and a density of approximately 2.5 g / cm '. This plate was then tested in a nickel-zinc target. The discharge voltage at 2 A was essentially constant for about 7 Ah and was above I fi V.

! Example 2

49.50 g of silver oxide plus 0.50 g of polytetrafluoroethylene powder were added to 250 cm of propylene glycol with the modification as in Example 1 and processed so that the temperature was 93 ° C. The finished electrode material was etched to a thickness of * / _m mm . Two pieces of this material were then cut to an electrode size of approximately 6.4 x 10.2 cro. Between the

=> rolled electrode halves was then sandwiched an expanded metal mesh made of silver was inserted. The finished electrode was at 103 bar to a final thickness of 030 mm pressed. The electrode was then in a Sflberoxid-Ziak-PrufzeDe used. The discharge ο voltage at 12 A was essentially for about 3 Ah constant and was above 1.4 V.

Example 3

98.0 g of metallic silver and 2.0 g of Polytetrafluor-Ijäthylen-Putver urden on a large Waring mixer as in Example I in 4000 cm ^J Stoddard Solvent and mixed. The procedure of Example I was continued until the material was rolled out to a thickness of 0.56 mm, whereupon electrodes of approximately 30.5 X 10.5 cm were punched out of the material and covered with an expanded metal middle layer of silver at 68, 5 bar pressure were pressed. EHe resulting electrode had a density of 4.2 g / cm 'and a thickness of about 0.7 mm.

Example 4

94g cadmium oxide, 5 g silver powder and 1 g porytetrafluoroethylene purver were mixed as in Example I. The resulting panels became 7.6 X 7.6

yes-EIektroden cm stamped and two such electrodes with an expanded metal layer of silver at 141 kg / cm ^z compressed pressure. The resulting cadmium oxide electrode, the density of which was 3.1 g / cm ', was in a nicket cadmium battery with 20 Ah nominal

r. capacity checked. The discharge voltage remained at 5 A. essentially constant for about 20 Ah and was above 1.2 V.

Example 5

94 g of mercury (II) oxide, 5 g of graphite powder and 1.0 g of Potytetrafluoroäthylen were processed as in Example I into sheets of material approximately 0.5 mra thick. Electrodes approximately 5.1 cm in diameter were then punched from this material; between

4ΐ two of the rolled foils became an expanded metal mesh Inserted from nickel. This structure was then at 1380 bar using an electrode with a Final thickness of 0.76 mm and a density of approximately 8 g / cm 'pressed. The electrode was then in a

to mercury-cadmium battery used. The discharge voltage at 0.1 A remained _{essentially constant for about 2 V 2} Ah at over 0.8 V.

Bet game 6

592.5 g CuCl ₂ , 7.5 g polytetrafluoroethylene powder and 150 g graphite powder were mixed for 2 minutes with 2800 cm 'of the petroleum solvent described above, after which the mixture was vacuum-extracted to a residual solvent content of 480 g.

«Ο The resulting cake was between two rollers with a scan width of 93 mm repeated in six Passes ground, the extruded material was folded after each pass after rotation by 90 °. The resulting tape was then

1.5 see rollers with a gap width of 3.05 mm rolled, the material thus extruded cut to 20.3 cm X 1.2 m and this material in strips of 12.7 cm X 1.2 m cut, each with a

Gap width of 1.02 mm was passed through rollers. The material was then at 103 bar to a final thickness pressed by 0.25 mm. The resulting strips were air dried at 55 "C,

Example 7

A nickel mixture containing 2.418 kg of active material in the form of hydrated Nickel hydroxide with a surface area of approximately 100 nr / g and a particle size of approximately 0.2 micrometers was made with 0.608 kg of powdered graphite (average 0.5 micrometer) and 0.030 kg of dry Mixed polytetrafluoroethylene powder. the Solid components were mixed with organic turpentine oil substitute and placed on a kneader, which is a uniform, homogeneous mixture of active nickel hydroxide, graphite and polytetrafluoroethylene powder formed. The homogeneous material was then removed from the kneader and a calender grinder fed, on which the polytetrafluoroethylene powder after several calendering passes took the form of long, continuous strands. The first calender pass resulted in the formation of a continuous sheet of nickel approximately 0.9 mm thick.

Electrodes approximately 7.0 X 5.1 cm in size were cut from this 0.9 mm thick material. Two such pieces were then placed in a press tool on both sides of a nickel grid in the form of expanded metal screen material, foil or other appropriate structure and pressed together, the total force being 350 kN. To make it easier to remove from the tool, the electrode was _{wrapped in a V 40} mm thick layer of paper before pressing. After removal from the tool, the finished electrode had a thickness of 1.24 mm including V ₂₀ mm of paper wrapping, which corresponds to a total electrode thickness of 1.19 mm. The average density of this electrode excluding the metal grid material was 1.83 g / cm \ The nominal capacity of this plate corresponds approximately to a battery with 1.02 Ah. The behavior of these electrodes in nickel-zinc and nickel-cadmium batteries was found to be excellent. The discharge voltage after 4 hours of loading at 2 A was 1.2 V.

Example 3

There was a starting mixture of 3.54 kg of hydrated Nickel hydroxide, 0.91 kg of graphite and 0.09 kg of polytetrafluoroethylene powder with the in example 7 are used.

The electrodes were made as in Example 7 with the modification that the first panels were made before Presses were approximately 0.5 mm thick. Then pieces of the active mixture were made from about 10.3x7.79 cm punched. Between two pieces a metallic grid was inserted. The pieces were wrapped in paper as in Example 7. the Active material pieces with the grid were then placed in a tool and at a total force of 1.2 MN, a finished electrode 0.5 mm thick was obtained. 58 such plates were then / u assembled to a nickel-zinc battery with a nominal capacity of 80 Ah and a cell with a very high charging current characteristic. This cell showed when discharged at high Amperages an excellent behavior. At πner Discharging the battery with 10 C (800 A) resulted in a discharge corresponding to about 75% of the Capacity (60 Ah) to an end point of 1 1 V.

Example 9

The battery plates of Example 7 were here punched into round electrodes, at the same pressure formed and as positive plates in a Nickel-Was-Ki hydrogen battery used. The behavior was excellent. Over 600 cycles of 2-hour discharge there was no noticeable decrease to 70%.

Example 10

592.5 g Ni (OH) ₂ , 7.5 g polytetrafluoroethylene powder with 150 g graphite powder were mixed for 2 minutes with 2800 cm 'liquid Shell SoU, after which the mixture was vacuum extracted to leave 1250 g cake. The resulting cake was ground between two rollers with a gap width of 9.5 ma /, the material being passed through the salts repeatedly in six passes and the extruded material being folded below 90 ° after each pass. The resulting strip was then rolled between rollers with a gap width of 3.05 mm and the extruded material was cut to a size of 203 cm × 1.2 mm. Each of these 12.7 cm strips were passed through rollers with a gap of 1.02 mm. After drying, the material became

jo pressed at 103 bar to a final thickness of 0.25 mm. The strips obtained were air-dried at 66 ° C. and used in nickel-cadmium batteries.

Example 11

π The final electrode mixes contain weight% Polytetrafluoroethylene, 6% by weight graphite and 93% by weight nickel hydroxide. The cathode was in a Nickel-hydrogen test cell is used, the behavior of which is summarized in the following table.

Behavior after 100 cycles with C / 2 discharge 70%

Time, minutes Voltage, V Example 12 , 41 0 , 29 12th , 20 24 , 22 36 , 19th 48 , 16 60 , 10 72

50 g lead oxide and 0.25 g polytetrafluoroethylene were mixed with turpentine oil substitute as in Example 1 and filtered on a Buchner funnel, whereupon the filtered mixture was processed by 30 minutes' rollers to a size of about 15.2 x 12.7 cm. The lead oxide was then pressed Polvtetrafluoräthylen mixture _Μ in a tantalum Strcckmetallgittcr, cut and then a 5.1 X 5.1 cm pieces and pressed at 18 kN, pressed again, and was air dried. The electrode obtained in this way was discharged against a Blcianode in sulfuric acid (6n). Here> ci were about 10 mA / cm ² at 2 V and 5 '.) MA / cm ² at 1.75 V could be removed.

For electrodes where Polytetrafluoräthylcn-Powder as a binding agent for various active May

Riaiien serves, the volume of polytetrafluoroethylene with respect to the density of the active material is a variable ratio. The determination of which is useful in order to obtain a smaller weight percent range of polytetrafluoroethylene within the above range of 0.1 to 3 for a given active material % define. For example, in the case of lead oxide, since this has a higher density than zinc oxide, a lower percentage by weight of polytetrafluoroethylene than in the case of zinc oxide can be used, while both percentages of polytetrafluoroethylene continue to be in the stated range of 0.1 bN ? I wt. lie. For example, 0.1 to 1% polytetrafluoroethylene can be used for clothing dioxide and

Zinc oxide 1.5 to 3% polytetrafluoroethylene used with each of these low percentage ranges being based on the weight of the polytetrafluoroethylene represents the total weight of polytetrafluoroethylene and active material. Cedmium dioxide has one greater density than zinc oxide but lower density than lead dioxide. You can use polytetrafluoroethylene for cadmium oxide use in this way in a range of 0.5 to 2% by weight.

As shown in the examples, the mixture can be used to form the hydrophilic electrode body you several electrochemically convertible active materials and electrically conductive Contain fillers.

Claims (2)

Patent claims: I _r process for the production of hydrophilic electrodes which contain electrochemically convertible, active material, wherein powdered, active material is mixed with powdered Polytefarafhioräthylen in an excess of a Gteitmirtefe to a homogeneous mixture, by filtering a predetermined part of the lubricant M> from the mixture is removed, the thickness of the filtered mixture is reduced by the action of shear forces, and in this way a sheet material of fibrillated polytetrafluoroethylene is formed and the remainder of the lubricant is removed from the sheet material and in this way the electrode is formed, characterized in that one dry polytetrafluoroethylene is used in an amount not exceeding 0.1% to not more than 3% of the total weight of the active material and the polytetrafluoroethylene, that a non-aqueous lubricant is used, that the filtered mixture is subjected to the action of the shear forces and is removed s of the remainder of the lubricant from the sheet material keeps the temperature of the filtered mixture or sheet material below the Siate temperature of the polytetrafluoroethylene. 2. The method according to claim I, characterized in that that you also use filler in the production of the »homogeneous mixture, which has a higher electrical conductivity than the active material.